The invention relates to a longitudinally adjustable connecting rod, comprising a piston part to be attached to the piston of an internal combustion engine and having a piston part skirt, and a shaft part to be attached to the crankshaft of the internal combustion engine and having a shaft part skirt, the piston part skirt and the shaft part skirt being telescopically connected together.
Connecting rods serve to convert the vertically reciprocating movement of the piston of an internal combustion engine into a rotational movement of the crankshaft. For this purpose they have an upper end with a first eye, which is supported on a transverse axis in the piston, and a lower end with a second eye, which is supported about the crank pin of a crankshaft. Conventional connecting rods have a fixed length, so that a specific combustion chamber volume, defined by the piston in the cylinder, is associated with each angular position of the crankshaft.
With longitudinally adjustable connecting rods on the other hand, the distance between the attachment to the piston and the attachment to the crankshaft can be adjusted, so that the correlation between the crankshaft angle and the combustion chamber volume can be varied. The longitudinal adjustment is used, in particular, to adjust the engine compression ratio according to the operating conditions.
A longitudinally adjustable connecting rod of the type referred to in the introductory part is disclosed by U.S. Pat. No. 4,195,601. It comprises a piston part, which is attached to the piston of the internal combustion engine, and a shaft part, which is supported about the crank pin of the crankshaft. The two parts each have skirts extending in the axial direction of the connecting rod, which are telescopically connected together. By means of an expensive structure of intermeshing sleeves, pins and rings, chambers are formed between the piston part and the shaft part, the volume of which chambers varies according to the expansion of the telescopic connection. A part of the chambers can be subjected to pressure by way of ducts for a hydraulic medium running through the crankshaft and the shaft part. An extension or shortening of the telescopic connection can be achieved according to the level of said pressure in relation to the pressure acting on the piston in the combustion chamber. The disadvantage with this system is that relatively high pressures of the hydraulic medium must be applied in order to move the connecting rod. A fixing of the position of the connecting rod is achieved by closing the ducts for the hydraulic medium. This, however, places the hydraulic system under a high static pressure, which places correspondingly high demands on the seals.
U.S. Pat. No. 4,140,091 also discloses a similar system. Here too, a longitudinal adjustment of the connecting rod is produced by the application of a hydraulic pressure, the connecting rod being designed according to the principle of a piston rod in a cylinder.
WO 95/08705 furthermore discloses a longitudinally adjustable connecting rod, in which the upper end of the connecting rod is attached to the piston by way of an eccentrically supported disk. A rod connected to the disk, to the piston and to the connecting rod furthermore causes the eccentric disk to rotate as a function of the crankshaft angle. In this system, therefore, there is a fixed relationship between the crankshaft angle and the effective length of the connecting rod. It is not possible to adjust the connecting rod as a function of the operating condition of the engine.
Against this background, it was the object of the invention to provide a connecting rod of adjustable length, having a simplified construction and greater operating reliability. This object is achieved by a connecting rod having the features of claim 1. Advantageous developments are contained in the subordinate claims.
The connecting rod accordingly comprises a piston part, which is to be attached so that it can perform a (swivel) movement to the piston of an internal combustion engine, and which has a piston part skirt extending in an axial direction of the connecting rod. It furthermore comprises a shaft part, which is to be rotatably attached to the crankshaft or the crank pin of the crankshaft of the internal combustion engine and has a shaft part skirt extending in an axial direction of the connecting rod. The piston part skirt and the shaft part skirt are telescopically connected together, so that a longitudinal adjustment of the connecting rod can be achieved through shortening or extension of said telescopic connection.
The connecting rod is distinguished that an arresting device is provided, by means of which the piston part skirt and the shaft part skirt can be mechanically arrested in at least two different positions relative to one another. Arresting the piston part skirt and the shaft part skirt in relation to one another produces a mechanical connection, which absorbs the large axial forces acting on the connecting rod. This has the advantage that these forces no longer lead to a high static pressure in a hydraulic medium, the back-pressure and density of which are bound to ensure a constant connecting rod length. Accordingly there is no need to provide particularly pressure-resistant hydraulic chambers and seals, which are expensive to manufacture. Furthermore, the use of arresting devices has the advantage that these can be operated more flexibly, so that an adjustment of the length of the connecting rod is not just possible solely at crankshaft angles predetermined by the design.
The telescopic connection between the piston part and the shaft part is preferably achieved in that one of the skirts, particularly the piston part skirt, comprises an axially extending guide chamber, in which the second skirt (shaft part skirt) is displaceably supported. The second skirt therefore constitutes a type of piston rod, which is supported in the guide chamber of the first skirt. The cross section of the guide chamber may be circular or angular, especially rectangular. The design of the telescopic connection explained has the advantage firstly that it is easy to manufacture, and secondly that it is particularly stable and functionally reliable.
According to a preferred development of the invention, the arresting device comprises at least one pin which is supported in the piston part skirt or in the shaft part skirt so that it is displaceable transversely to the skirt axis. In this case, the pin is displaceable between a locking position and an unlocked position, the pin in the locking position engaging in a recess on the other skirt, whilst in the unlocked position it does not engage in said recess. In the locking position the pin forms a coupling between the two skirts, so that the facility of the skirts for axial displacement is canceled. All forces acting on the connecting rod in an axial direction are transmitted from one skirt to the other by way of the pin. The necessary stability of this coupling can be assured through a corresponding design of the pin and its guides. Supporting the pin transversely to the skirt axis has the advantage that it does not have to be displaced in opposition to the loads acting in an axial direction of the connecting rod. The pin can therefore be shifted between the locking position and the unlocked position by applying relatively small forces.
In the last-mentioned development, the pins are preferably preloaded into one of their two possible positions by an elastic element or a spring. This is preferably the locking position. Preloading by means of a spring means that for external movement of the pin, force has only to be exerted in one direction. In the opposite direction, on the other hand, the motive force is applied by the spring, which can be compactly arranged directly on the pin. Preloading, especially into the locking position, has the advantage that the connecting rod tends to assume a stable condition at the arrested length. This increases the reliability of the system in response to faults in the externally acting adjustment mechanism for the pins.
According to a further development of the arresting device with pins and recesses, the piston part skirt or the shaft part skirt has at least one bore extending transversely to the skirt axis and in which two pins are supported, so that they can each emerge in opposing directions from one end of the bore. This arrangement has the advantage that an interlock is achieved between the skirts of piston part and shaft part, which is symmetrical in relation to the axis of the connecting rod, so that force is transmitted uniformly.
In the last-mentioned arrangement a spring, which pushes the pins apart, can be situated between the two pins. The spring therefore tends to push the pins in a direction out of the bore and into a recess in the other skirt, that is to say into the locking position.
Electrical or hydraulic devices are preferably provided, which are capable of moving the pins from the locking position into the unlocked position. The arresting mechanism can therefore be canceled through actuation of these devices, in order to adjust the length of the connecting rod. A movement of the pins in the opposite direction, that is to say from the unlocked position into the locking position, can likewise be performed by said electrical or hydraulic devices, or the pins may be preloaded by a spring acting in this direction, as referred to above.
For accomplishing a hydraulic actuation of the arresting device, preferably at least one feed duct for a hydraulic medium is provided in the shaft part, the feed duct being in fluid connection with a recess when piston part and shaft part are in a relative position such that a pin of the piston part engages or is capable of engaging in said recess. In the following, it will be assumed without restriction of universality that the recess is arranged in the piston part and the pin in the shaft part. When the pin of the shaft part skirt engages in the recess, an interlock exists between the piston part skirt and the shaft part skirt. This interlock may be canceled through the connection of the feed duct to the recess, by delivering hydraulic medium (fluid or gas) under pressure by way of the feed duct, so that a corresponding pressure is built up in the recess. This pressure in the recess acts on the pin located there and forces it to move out of the recess, that is to say into its unlocked position. As soon as the pin has fully left the recess, the interlock between the skirts is canceled, and the length of the piston rod can be adjusted.
According to a further development of the invention, the piston part and the shaft part may have bearing surfaces, which are designed and arranged so that they are in contact and transmit forces in an axial direction, when the piston part and the shaft part assume a lockable relative position. This has the advantage that the forces to be transmitted in an axial direction of the connecting rod do not have to be transmitted from the piston part to the shaft part solely by the arresting device, said bearing surfaces instead assuming a considerable part of the load in this case. Accordingly, the arresting device may be of a lighter and simpler design.